Springback is a phenomenon always present in the bending of metal. Bending operations for sheet metal are typically carried out by the use of presses broadly classified by the source of power as hydraulic or mechanical. Certain alternatives are available when using hydraulic presses to control springback, within tolerable limits, because of the lower strain rate involved. However, more efficient and rapid production can be achieved with mechanical presses which use much higher strain rates resulting from high speed ram movement.
The final shape of sheet metal parts formed by mechanical press bending depends importantly upon the control of springback. Springback is the natural tendency of the material to revert to its original shape after the bending force has been removed. It has been generally believed heretofore that the springback is proportional to a certain group of parameters which include the bending radius, the thickness of the product material, and the hardness of the material. It has been conventional for tool designers to correct such springback by (a) over-compensating through an overbend whereby the product will relax to a shape that is precisely desired upon relief of the bending force, or (b) restriking the material in the same die at the same bend point to encourage the material to more closely conform to the desired die configuration. To facilitate overcompensation, tables of data resulting from incremental changes in springback with variances in the material thickness, hardness and bend radius have been generated. However, due to the numerous variables that seemingly affect mechanical press springback, such tables of data have been limited to simple bends, as in a V-shape.
Springback thus remains a problem in the pressing of mild steel into complex shapes. With the advent of high strength, low alloy steels having yield strengths in excess of 50,000 psi in relatively thin sections, it has been found that projecting and compensating for springback, based upon various physical characteristics of the material, does not work. It appears that the compound effect of higher material strength and typically higher mechanical press speeds, to form the material, cause considerably greater springback than that which is often encountered in producing parts made of conventional mild steel.